Mark flaky flicker assertion

This is currently flaky as sometimes all the surfaces become invisible
when going into AOD. To unblock presubmit, this will be marked flaky
until a fix is submitted.

Test: atest
  FlickerTestsIme2:com.android.server.wm.flicker.ime.ShowImeOnUnlockScreenTest
Bug: 338178020
Flag: NONE
Change-Id: I652b470407c7efbf88f9ea8b48a4491ae0b726e9